WO 07/036930 in the name of the present applicant describes an object adapted to float in air when filled with a gas having lower specific gravity than air, the object including a hollow enclosure having an inlet coupled to a pressure adjustment means for regulating pressure of gas within the enclosure so as to ensure that a buoyancy force alone of the enclosure counteracts a weight of the object. A number of general approaches for regulating gas pressure within the enclosure are suggested. Thus, in accordance with one approach the enclosure is part of a buoyant platform, which supports an object in space and it is suggested to couple a gas connector to an inlet of the enclosure via an adjustable pressure valve, whereby gas may be fed at a controlled pressure to the buoyant platform. By such means the buoyancy of the platform may be adjusted so as to exactly counteract the combined weight of the buoyant platform and the attached object, thereby allowing the buoyancy of the platform to be adjusted when different objects are attached.
The adjustable pressure valve may be set using trial and error so that the gas pressure is exactly correct to achieve buoyancy for an object anchored to the support structure or integral therewith. Alternatively, the user may be informed of a suitable gas pressure to supply to the buoyant support structure, based on the buoyant gas being used, expected ambient conditions, and the mass of the object to be supported.
It is also suggested to adjust gas pressure using a flexible diaphragm within the hollow support structure and to adjust the effective gas volume within the hollow support structure by displacement of the flexible diaphragm.
While such approaches work when conditions are clearly defined and not subject to constant fluctuation, they are not suited for maintaining a constant height when ambient conditions vary. Particularly, the devices described in WO 07/036930 are apt to wander unless tethered, owing to air currents for example. Moreover, the need to calibrate the pressure valve according to different ambient conditions and objects may not be convenient or may not be possible with sufficient accuracy. It is suggested to provide a remote controlled propeller so as to allow controlled movement in space of the supported object.
U.S. Pat. No. 7,341,224 discloses a miniature robot surveillance balloon system having an electronic processor subsystem that controls vertical movement. Buoyancy may be controlled using a gas cylinder carried by the balloon assembly and containing a gas that is lighter than air, such as helium. In another embodiment, the balloon is pre-inflated so as to obviate the need to attach a gas cylinder to the balloon assembly. In such case, the only way that the effective buoyancy of the balloon may be increased is to jettison ballast. And regardless of whether or not an external gas cylinder is provided, the only way that the effective buoyancy of the balloon may be decreased is to release gas from the balloon. Such an approach is acceptable where the principal requirement is to raise the surveillance system to a predetermined height and then bring it back to earth. However, it is not acceptable where servo-assisted altitude regulation is required because once the helium is released in order to reduce buoyancy there is no way that the buoyancy can be subsequently raised since the quantity of helium cannot be increased. This is an inherent problem with using the helium to regulate the buoyancy as distinct from merely providing the buoyancy. Furthermore, the need to carry a gas cylinder is not practical for portable devices where the mass of the gas cylinder may well exceed that of the rest of the system.
US 2006/0065777 discloses a density control buoyancy system having a processor controlling valves, an inlet valve allowing air into a compartment to compress lifting gas and an outlet valve for releasing air from the compartment to decompress the lifting gas. An airship shown in FIG. 3A has a rigid outer hull containing lifting gas and which contains an inner flexible compartment that contains air that may be regulated to control buoyancy. A controller controls the functions and operations of the inlet valve and/or pump and the outlet valve and/or pump for regulating air flow.
The controller allows equilibrium to be maintained once the airship has reached a required height under manual control of a pilot, but it is not capable of automatically raising the airship accurately to a required height. Moreover, the principle of operation is based on density control, whereby differential pressure between a lifting gas (helium) inside the hull and air inside the flexible compartment is used to regulate air flow into or out of the flexible compartment in order to maintain equilibrium. It is clear that while such an approach may be feasible for an airship where differential pressure over an extended height range may be measurable, it is not feasible for use at limited height ranges where pressure gradients are negligible. To put this in perspective, pressure at sea level is 101,325 Pa, and at 5 meters is 101,253 Pa. At a height of 1 km, it is 87,836 Pa and at a height of 10 km it is 24,283 Pa. Thus, while pressure gradient between sea level and 1 km is significant, the pressure difference over a height difference of 5 m is a mere 72 Pa, which is probably too low to serve as a practical error signal for a servo control system. Certainly, the difference in atmospheric pressure at 3 m (101,282 Pa) and 3.5 m (101,274 Pa) is a mere 8 Pa and it is clear that this cannot serve as a practical feedback signal. Likewise, once the airship has reached a target altitude, to maintain it at this altitude to within a resolution of ±1 m based on differential pressure feedback is inconceivable. It will equally be appreciated that other ambient conditions such as temperature that might be used directly or indirectly to provide a servo error signal at exalted altitudes are not suitable at low absolute or differential altitudes.
Furthermore, controlling height based on pressure variations allows height to be maintained relative to sea level, but it does not accommodate variations is terrain. This is not a problem for an airship which climbs to a sufficiently high altitude to be well clear of tall buildings and mountains. But it is not suitable for accurate height control relative to ground.
It is thus clear that US 2006/0065777 is not amenable to lifting an object to a set height in a confined space such as a room or to maintain the object at the set height to within an accuracy of less than 1 meter.
US 2008/0265086 discloses a lift gas ballast system for a tethered aerostat used in an airship has lift gas ballast tank that is disposed on the ground, and connected to tethered aerostat through the double slip ring and the hollow feed tube.
US 2008/0135678 discloses an airship for transporting passengers and cargo, having a controller which regulates flow of first gas into and out of a compartment to actively control the ascent and descent of airship.
U.S. Pat. No. 5,782,668 discloses a balloon for advertising having an internal light connected to a fixed power supply and which is disconnected if the balloon deflates or its surface deforms.
U.S. Pat. No. 3,839,631 discloses an automatically equilibrating inflated suspended object that is lighter than the surrounding medium. Equilibrium is automatically achieved by means of a flexible tether extending from a fixed elevated point to the object. Vertical movement of the object varies the portion of the weight of the tether supported by the object until the object supported weight of the tether equals the lift.
FR 2 372 075 discloses a helium filled distress balloon whose altitude is stabilized using air, and which is fitted with a transmitter.